The present invention relates to PCM signal demodulating circuits, and more particularly to a PCM signal demodulating circuit used in a digital audio system.
In a digital audio system, an audio signal is sampled at a predetermined sampling frequency to obtain sampled values. The sample values thus obtained are then digitally coded.
In these digital audio systems, a variety of coding formats and sampling frequencies are employed. For instance, in a format used in a system using a digital audio disk as the recording medium, the sampling frequency is conventionally 44.1 KHz. In a format employed in a consumer PCM processor or the like, the sampling frequency is typically 44.056 KHz. In a format employed in an operational PCM recorder or the like, the sampling frequency is usually 50 KHz or 50.4 KHz.
A plurality of different types of data sampled at different sampling frequencies as described above may be handled by a single demodulating circuit if sampling frequency conversion is carried out. The sampling frequency conversion preferably is performed in the step of converting the input signals to digital form so as to prevent deterioration of the signals. However, this method is disadvantageous in that, in order to prevent an increase in the circuit size of the demodulating circuit, the ratio of sampling frequencies before conversion must be a simple integer ratio, for instance 50.4:44.1 (=8:7).
In order to overcome this difficulty, a demodulating circuit has been proposed which is designed so that data is processed according to the sampling frequencies of the various formats and an error correcting system or the like, and the audio signal is demodulated using a single D/A (digital-to-analog) converter. However, in this demodulating circuit, if, for instance, the sampling frequency is 32 KHz, an audio signal component occurring in the output of the D/A converter and an unwanted component arising due to sampling are distributed as indicated by a solid lines a and b in FIG. 1; that is, the band of the unwanted component is higher than 16 KHz. If the sampling frequency is 44.1 KHz, the audio signal component and the unwanted component are as indicated by the broken lines c and d in FIG. 1; that is, the band of the unwanted component is higher than 22.05 KHz.
Accordingly, in order to eliminate unwanted components from the output of the D/A converter, it is necessary to provide a plurality of low-pass filters having different cut-off frequencies and to selectively operate the low-pass filters according to the sampling frequency employed. Therefore, the demodulating circuit is necessarily large in size. Furthermore, the circuit has a high manufacturing cost.